"Durable foodstuffs" include cereal grains (such as wheat, barley and oats), pulses (including legumes, lentils, soybeans, peas and peanuts), rice, coffee and cocoa beans, and the like. Such foodstuffs may be stored in a bunker storage. A bunker storage was originally formed by excavation of earth but now is usually an above-ground structure consisting of a flat floor of earth or concrete, which may be covered with a water-impervious layer (such as paint, plastic or bitumen), bounded on three sides by walls of earth, sheet metal or concrete. The fourth side is left open when the durable particulate foodstuff (which, for convenience, will be regarded as grain in this specification) is being loaded into the storage. Grain is placed on the floor so as to form a pile within the walls. It is then covered with plastic sheeting and the individual sheets are joined together to form a continuous cover that is secured to the walls to provide an almost sealed envelope surrounding the grain pile. If the walls are of earth, the edges of the cover may be buried in the earthen wall. If the floor is covered with a membrane, the edges of this membrane and of the plastic sheeting may be clipped or otherwise joined together, prior to being secured to the walls. Sometimes the plastic cover is itself covered with earth. The fourth side of the bunker may be closed off at this stage by a retaining wall of concrete or of other suitable material, or the top cover of the grain may be sealed to the floor. Good sealing of the bunker prevents the entry of pests and allows fumigation to be carried out to disinfect the grain held in the bunker.
Unfortunately, when grain is stored in such bunkers, some parts of the grain bulk may become excessively moist during the storage and may consequently suffer degradation through moulding and fermentation. The sources of this moisture may be (a) moisture originally present in the grain, (b) metabolic moisture, derived from the activities of pests, microorganisms and natural processes in the grain, (c) water from leaks in the grain cover, and (d) moisture from the ingress of humid air. The moisture usually collects close to the top surface of the pile of grain, where it may cause a consolidated region of grain to form. This consolidated region of moisture-affected grain usually occurs from 3 to 6 inches below the surface of the grain. When the grain is to be retrieved from the bunker, this consolidated region (which is often spoiled) has to be removed first. The remainder of the grain is then extracted from the bunker. Hitherto, the removal of moisture from grain and other stored, particulate, durable foodstuffs has required the application of hot air or the like, provided by external sources of energy.